Lecture 7

Question 1

Explain and describe the 2 polypeptide chains that make up most TCR’s on T cells. What are the 2 chains that only a few T cells express and why they are present?

Answer 1

Most TCR’s are composed of “Alpha” and “Beta” polypeptide chains that are each contain a C (constant) and V (variable) regions

Some expressed TCR’s are composed of “Gamma” and “Delta” bc they have a broader specificity for unconventional Ag’s such as HSP’s and Phospholipids

Question 2

True or False:

TCR’s can recognize native antigens as well as processed antigens. explain.

Answer 2

False

TCR’s CANNOT recognize native antigens, they can only recognize processed Ag’s presented in MHC molecules

Question 3

The genese encoding for TCR polypeptide chains are members of the __ _____ ______.

Answer 3

Ig super family

Question 4

What exactly composes a T cell Receptor complex (3 units)

Answer 4

a T cell Receptor complex is composed of an Ag receptor TCR with it’s alphabeta OR gammadelta dimer (depending if it is a conventional TCR or not) in association with CD3.

(CD3 subunit on one side, TCR in the middle, and another CD3 subunit on the other side

Question 5

Explain what subunits the CD3 signalling complex is composed of and how it contributes to the over all TCR complex.

Answer 5

CD3: has 2 separate subunits that sit on either side of the TCR in order to form the 3-part TCR complex

One subunit of CD3 is a sigma and delta chain

Another subunit of Cd3 is a gamma and epsilon chain

Question 6

what encodes human leukocyte Ag and are critical to Ag presentation?

Answer 6

Class I and II of polymorphic MHC genes

Question 7

Describe class I and II of MHC genes

Answer 7

Class I: (HLA-A, -B, -C) encode a polymorphic chain that combines with Beta1-microglobulin and is expressed on the surfaces of all nucleated cells

Class II: (HLA-D) encode molecules composed of 2 dissimilar polymorphic polypeptide chains (an alpha and beta chain) which contribute to the peptide-binding groove

Question 8

which class of MHC do the following genes belong to? what do they encode for?

HLA-D

HLA-A, -B, -C

Answer 8

Class II: (HLA-D)
Encode for HLA-DP, -DR, and -DQ which are composed of 2 polymorphic polypeptide chains (alpha and beta) that contribute to the peptide-binding groove

Class I: (HLA-A, -B, -C)
Encode for a polymorphic heavy chain with a binding groove for Ag peptides which combines with the non polymorphic Beta1-microglobulin

Question 9

Which regions of MHC molecules are the peptide-binding domains? Delineate the amino acid residue ranges that each of them bind to and what it is about their structure that dictates the size of peptides it may bind to.

Answer 9

The polymorphic regions are the peptide binding domains of MHC molecules (class I and class II)

MHC class I binds to peptides ranging from 8-10 due to it’s “closed” structure

MHC class II binds to peptides ranging from 10-20 due to its “open” structure

Question 10

Which class of MHC molecules are expressed by professional antigen presenting cells?

Answer 10

BOTH! because all cells must express class I

MHC class II is only expressed by Professional APC’s however the professional APC’s must also express MHC I molecules

Question 11

What modulates the expression of MHC class I and/or class II molecules?

Answer 11

cytokines

Question 12

Briefly describe the MHC class I, pathway, and type of effector T cell that is used to respond to Intracellular microbes. Do the same for Extracellular microbes.

Answer 12

intracellular microbes in infected host cells move as a complex to the surface of the cell via the “endogenous pathway” and then bind to “MHC class I molecules” to be presented on the cell surface to CD8+ (cytotoxic T cells)

MHC class II pathway begins with the uptake of Extracellular microbes into the APC where they are processed, attached to MHC class II molecules, and presented to CD4+ (Helper T cells)

Question 13

What is CD40? (where it is found and it’s function)

Answer 13

a costimulatory protein found on pro APC’s that is required for their activation

the binding of CD40L (ligand) expressed on T cells to CD40, activates APC’s

Question 14

where will a microbe end up and what cell will interact with it if it enters through the skin/GI tract/respiratory tract OR if it enters through the blood?

Answer 14

microbes that enter the body via the skin/GI tract/respiratory tract will be captured by dendritic cells and brought to a lymph node

Microbes that enter the body via the blood are captured by APC’s in the spleen

Question 15

State whether it is a tissue resident dendritic cell or an activated dendritic cell that expresses the following qualities.

Expression of Fc receptors, Mannose receptors:

Expression of molecules involved in T cell activation (B7, ICAM-1, IL-12):

Longer half life:

more surface molecules:

Answer 15

Expression of Fc receptors, Mannose receptors: Tissue resident DC

Expression of molecules involved in T cell activation (B7, ICAM-1, IL-12): Activated DC cells

Longer half life: Activated DC cells

more surface molecules: Activated DC cells

Question 16

compare the primary functions of a resident tissue DC and an activated DC.

Answer 16

resident tissue DC: antigen capture

Activated DC: Antigen presentation to T cells

Question 17

True or False:

One DC cell is able to conduct multiple simultaneous docking with more than one T cell and will present Ag’s to thousands of T cells before it dies. explain.

Answer 17

True

Question 18

NK cells produce IFN-gamma during innate immune reactions to microbes OR IFN-gamma is produced by T cells during adaptive immune reactions. Describe what IFN-gamma stimulates and what IFN-gamma alongside type I interferons stimulate.

Answer 18

IFN-gamma: stimulates class II MHC expression on APCs and thus enhances the activation of CD4+ T cells

IFN-gamma alongside type I interferons: stimulate the expression of MHC class I molecules and thus stimulate the activation of CD8+ T cells

Question 19

describe the Class I MHC pathway (include TAP and ERAP)

Answer 19

A virus infects a cell, viral protein is synthesized, and is then degraded by a proteasome after it if ubiquitinated

Then viral peptides from the cytoplasm are then brought to the ER and enter via TAP (transported associated antigen processing)

once in the ER, ERAP (endoplasmic reticulum associated peptidase) forms a peptide-class I complex with the peptide

This complex then leaves the ER, moves to the golgi, and is then exocytosed to be expressed on the surface

Question 20

Describe the Class II MHC pathway (include CLIP and HLA-DM)

Answer 20

Extracellular protein is phagocytosed into an APC and processed via fusion with a lysosome

The ER synthesizes a class II MHC molecule (WITH a CLIP protein to prevent binding in ER) and packages it into an endosome (from the golgi) to form an exocytic vesicle

The processed peptides and class II MHC molecules associate with one another in a vesicle with the help of HLA-DM (ensures that only the most relevant peptides for eliciting an immune response are what gets presented).

The newly formed Peptide-MHC complexes are then presented on the cell surface

Question 21

State the 3 major functions of HLA-DM

Answer 21

HLA-DM causes the dissociation of CLIP from the peptide binding groove of MHC II

HLA-DM stabilizes and prevents the degradation of the empty MHC II

HLA-DM facilitates the binding of Ag fragments to the open, stabilized binding groove

Question 22

State the type of Antigens that CD4+ T cells, and CD8+ T cells recognize (include which class of MHC molecules are associated with these as well.

Answer 22

CD4+ T cells recognize extracellular Ags.
MHC class II molecules display peptides that are taken up from the environment.

CD8+ T cells recognize intracellular Ags
MHC class I molecules present peptides from cytosolic proteins (created by the metabolic activities of an intracellular pathogen)

Question 23

True or False:

AlphaBeta T cells recognize only linear peptides that can be found associated with a cell or in their soluble form. explain.

Answer 23

False

AlphaBeta T cells recognize linear peptides (linear bc all conformation of the Ag is lost during enzymatic processing) HOWEVER AlphaBeta T cells ONLY cell-associated Ags.

AlphaBeta T cells recognize only MHC bound Ag peptides.

Question 24

We know CD4 T helper cells attach to MHC class II molecules and that CD8 T cytotoxic cells attach to MHC class I molecules. What determines this differentiation? (be specific)

Answer 24

Both CD4 and CD8 attach to the “non polymorphic” (Non variant) part of their respective MHC molecules

(this makes sense that they attach to the portion of the MHC molecule that does not “change” to ID a broad range of pathogens, bc it keeps CD4, CD8, MHC II, and MHC I organized respectively)

Question 25

Recognition of the peptide Ag by the TCR on a T cell is not sufficient to activate the cells. What else must occur to activate a T cell?

Answer 25

Co-stimulatory molecules expressed by APCs and CD4/CD8 Co-receptors (one of these not both) are also required to activate a T cell

Question 26

T cell activation leads to the production of IL-2, which triggers what?

Answer 26

IL-2 triggers clonal expansion of the specific T cell that was activated

Question 27

Th1 and Th2 are the 2 different phenotypes of T helper cells and their response is dictated by what?

Answer 27

the cytokines that they produce

Question 28

For Th1 and Th2, state whether they combat intra/extracellular microbes, which cells they aid, and which cytokine they produce to aid these cells.

Answer 28

Th1 T cells help resident tissue macrophages combat intracellular microbes AND help the development of cytotoxic T cells by secreting IFN-gamma (cytokine) Th2 cells help B cells develop into effector B cells (memory and plasma cells) which combat extracellular microbes by secreting IL-4 (IL-4 is important for B cell proliferation)

Question 29

For MHC class I and II molecules, explain how they are structurally different and which of their chains are glycosylated

Answer 29

Class I MHC molecules have a polymorphic alpha chain and a polymorphic Beta2 microglobulin chain. only their Alpha chain is glycosylated Class II MHC molecules have both a polymorphic alpha and a polymorphic beta chain. Both chains are glycosylated

Question 30

Describe the polymorphic and non polymorphic portions of MHC cells (you don't need to differentiate the 2 classes bc the polymorphic and non polymorphic portions of both classes behave in the same way)

Answer 30

The polymorphic portion of the MHC molecule is widely variable so that it can recognize a wide variety of Antigen peptides (these peptides sit in the polymorphic groove) The non polymorphic portion is what the CD4/CD8 portion of the Helper/cytotoxic T cell binds to in order to recognize that the appropriate type of T cell is interacting with the appropriate MHC class

Question 31

What is the function of the "anchor residue" on a peptide that is being presented by a MHC cell? how does this affect the specificity of the MHC cell?

Answer 31

it basically anchors the peptide to the MHC so that it may present the T cell contact residue to the TCR (forms a sandwich) A single MHC can present several different peptides that may have very different T cell contact residues. As long as the anchor residue of the peptide can anchor it to the MHC, the same MHC can bind to several different Ag peptides.

Question 32

State the 3 APCs and what their specific function is

Answer 32

DC's function to activate Naive T cells (only one that can do this) Macrophages function to activate Effector T cells (cannot activate naive cells) B cells function to activate Effector T cells (cannot activate naive cells)

Question 33

Distinguish the roles of B7, CD40, CD40L, and CD28

Answer 33

B7 is present on the surface of an APC, and interacts with CD28, which is on the surface of a T cell, to stimulate the growth and differentiation of the T cell CD40L is present on the surface of the T cell and interacts with CD40, which is on the surface of the APC, and signals for the activation/differentiation of the APC cell

Question 34

State which cells produce IFN-gamma for innate immune reactions and adaptive immune reactions respectively

Answer 34

Innate Immune reactions: IFN-gamma is produced by NK cells Adaptive Immune reactions: IFN-gamma is produced by Cytotoxic T cells

Question 35

Explain the organelle within which the respective MHC molecule will form a complex with the Ag peptide for the MHC I and MHC II pathways. (MHC II pathway has 3 steps)

Answer 35

MHC I pathway allows the MHC I molecule to complex with the Ag in the ER, then the complex moves to the golgi to be put into an exocytic vesicle MHC II pathway synthesizes the MHC II molecule in the ER with a CLIP peptide to keep it inactivated. Then the MHC II molecule with CLIP goes to the golgi to be put into an exocytic vesicle. Once in the vesicle, the processed peptide joins the vesicle, CLIP dissociates, and the MHC II molecule presents the peptide on the cell surface

Question 36

State the purpose of the CLIP peptide in the MHC II pathway AND state why CLIP peptide is not needed in the MHC I pathway

Answer 36

It prevents any of the "self peptides" (which there are a ton of in the ER, where the MHC II molecule is made) from binding to the MHC II molecule and being presented It blocks the MHC II molecule from binding to a peptide until HLA-DM can help the most relevant peptide be selected The MHC I pathway WANTS self peptides to be presented and destroyed (duh, MHC I works with CD8+ cytotoxic T cells which destroy cells that are already too far infected from intracellular pathogens)

Question 37

State the type of Cell(s) that MHC Class I and MHC class II molecules are present in.

Answer 37

MHC Class I molecules are found on all nucleated cells MHC class II molecules are found on APCs (DCs, B cells, Macrophages)

Question 38

Explain the concept of cross-presentation of Ag's by dendritic cells

Answer 38

Basically, DCs can still present extracellular Ags in association with class I MHC molecules to CD8+ cytotoxic T cells. This can occur bc DC's have both class I and class II MHC molecules and if a pathogen is phagocytized into the DC and LEAKS into the cytoplasm, then the DC can present the extracellular pathogen via the same class I MHC pathway it uses for intracellular pathogens.

Question 39

Explain what an immunodominant peptide is.

Answer 39

An immunodominant peptide is the "best available" epitope (peptide piece) that is selected to bind to the MHC molecule (either class). If you were to put the same protein into a cell, by the time it is processed and a peptide is selected, the immunodominant peptide will ALWAYS be the one that is selected to bind to the MHC molecule and be presented

Question 40

What the purpose of CD47?

Answer 40

``` it is a ligand that interacts with inhibitory receptors of NK cells to signal that the cell is healthy and does not need to be destroyed by the NK cell. (found on RBC's a lot bc they are not nucleated and therefore cannot express MHC class II molecules to signal that they are healthy to NK cells) ```